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What is sidereal time?

Explanation of sidereal time, or star time. New Year’s celebrations on Mercury. How Venus spins backwards and is the slowest rotating planet in the solar system.

When Earth is at position 1, it is noon at the red dot. One sidereal day later, Earth has rotated once and moved along its orbit (position 2). For it to be noon again, Earth has to continue rotating for another four minutes (position 3). Credit: Wikipedia

A sidereal day measures the rotation of Earth relative to the stars rather than the sun. It helps astronomers keep time and know where to point their telescopes without worrying about where Earth is in its orbit.

Every 24 hours, the Earth spins once around its axis and the sun loops around the sky. From noon to noon – or the time it takes the sun to return to its highest point in the sky – is how we define the days of the week. Astronomers call this a solar day.

But the time it takes for the sun to make one circuit around the sky and the time it takes our planet to complete one rotation is not the same thing. If you’ve spent your life thinking that 24 hours is how long it takes Earth to rotate, you might be in for a surprise.

In the time it takes the Earth to spin once about its axis, it also moves along its orbit by over 2.5 million kilometers. Because Earth has moved, the sun will not appear in the same part of the sky at the end of that rotation. To end up facing the sun again, the Earth has to rotate for another four minutes.

In other words, a solar day is how long it takes Earth to rotate once – and then some. A sidereal day – 23 hours 56 minutes and 4.1 seconds – is the amount of time needed to complete one rotation.

In this system, the stars always appear at the same place in the sky at the same time each sidereal day. Sidereal noon is when the vernal equinox – where the sun sits in the sky at the first moment of northern hemisphere spring – passes directly overhead. The four minute difference between sidereal and solar days can be seen by watching the stars rise four minutes earlier every night. If Vega is rising at 9 P.M. tonight, then it will rise at 8:56 P.M. tomorrow, and 8:52 P.M. the following night, and so on. As Earth travels about the sun, we see each star earlier and earlier.

Mercury has to orbit the sun twice from one Mercurian noon to the next. Credit: Wikipedia

Sidereal days are also how astronomers define the rotation periods of other planets. It helps isolate how quickly the planet is actually spinning from how fast it’s traveling about the sun. In most cases, like Earth, the difference between a solar day and a sidereal one is pretty small. But our solar system does have some notable exceptions.

Mercury’s rotation rate is two-thirds of its orbital period: a Mercurian sidereal day is 58 Earth-days while its year is 88. Because the sidereal day is a considerable fraction of the planet’s orbital period, an inhabitant of Mercury has to wait about 170 Earth-days from one noon to the next.

But this means that a solar day on Mercury is longer than its year!

One Mercury year is about one-half of a Mercury solar day. Imagine ringing in the year 2012 at midnight, and then gearing up for the next New Year’s celebration at noon!

Venus over the Pacific Ocean. Our closest neighbor in space and about the same size and density as Earth. But the sun only rises twice in a Venusian year. Plus Venus spins backwards, relative to other worlds in our solar system. Credit: Brocken Inaglory via Wikipedia

Venus is a particularly odd case. She goes around the sun faster than she spins on her axis: a 225 Earth-day orbit versus 243 to complete one rotation. This is why Venus is the slowest spinning planet in the solar system. At Venus’ equator, the planet is spinning at about 6 km/hr while Earth’s equator is hurtling along at nearly 1700 km/hr.

What’s more, Venus does this while spinning backwards. If there were ever to be a break in Venus’ stifling cloud layer, the native Venusians would watch the sun rise in the west and set in the east.

The backwards rotation makes Venus the only planet in the solar system where the sidereal day is actually longer than the solar one. The sun returns to it highest point in the sky before the planet has completed one rotation.

Combining all this together leaves Venus with a solar day that takes 117 Earth-days. Put another way, the sun only rises twice in a Venusian year.

Sidereal time measures the rotation of our planet relative to the stars. It allows astronomers to keep time without worrying about the motion of Earth around the sun. And it reveals some of the quirky motions of our planetary brothers and sisters. Next time your clock strikes noon, try and imagine what life might be like on a world where the sun moves backwards or doesn’t get a chance to set before the year is over. Turns out, such alien environments are right next door!

Christopher has a Ph.D. in astronomy from the University of California, Los Angeles. After eight years of searching for exoplanets, probing distant galaxies and exploring comets, Chris realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a 2013 AAAS Mass Media Fellowship to write for Scientific American, he left a research career at the U.S. Naval Observatory to pursue a new life writing about anything and everything within the local cosmological horizon. Since 2014, he's been working with Science News.